What astronaut Scott Kelly's 340-day stay on the ISS will tell us about the effects of space travel

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Along with all the experiments and research astronauts conduct on the International Space Station, the simple fact of them just being there is also providing crucial data to NASA about how space travel affects the human body. So, as Scott Kelly heads back to Earth today, what can we learn from his 340-day stay in the heavens?

NASA started monitoring Kelly’s vitals a full year before he blasted off for the ISS, and they’ll be tracking him for at least the next year once he’s back on Earth. Even cooler, NASA has also been monitoring his twin brother Mark (a former astronaut) back here on Earth, and they can now (almost) directly compare how a year in space affects the human body. Apart from cloning a guy and sending his doppelgänger up, this is the closely we can get to a control base for the effects of space travel.

Along with the actual technology and science of it all, the space agency notes that figuring out a way to keep humans healthy during prolonged space flight is also one of the biggest hurdles for a Mars mission. NASA notes that scientists have solid data about how bodies respond to living in microgravity for up to six months, but significant data beyond that timeframe had not been collected. At least, not until now.

Any Mars mission will probably take approximately three total years, with half that time spent on travel (there and back), and the other half spent exploring the planet. But everything from vision to bone density to the flow of blood within the body is affected by space travel. So tracking how Kelly’s body was affected will be the closest we’ve ever come to realizing how a human might handle a full-on Mars trip. It’s worth noting this isn’t the first time someone has spent a year in space (the Russians did it in the 1990s), though it is the first time scientists have closely tracked vitals at this degree to see the effects.

NASA says it’ll be 6-12 months until we see any hard, published data from Kelly’s tenure in space, but whatever they find, it’ll help us get one step closer to exploring our solar system.